The present invention relates to hi-current implanters used to implant ions in semiconductor wafer substrates in the fabrication of semiconductor integrated circuits on the substrates. More particularly, the present invention relates to devices for controlling the quantity of electrons emitted from a PFS (plasma flood system) arc chamber into an ion beam in order to reduce or control surface charges on a substrate during an ion implantation process.
In the semiconductor production industry, various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
Ion implantation is another processing step commonly used in the fabrication of the integrated circuits on the wafer. Ion implantation is a form of doping, in which a substance is embedded into the crystal structure of the semiconductor substrate to modify the electronic properties of the substrate. Ion implantation is a physical process which involves driving high-energy ions into the substrate using a high-voltage ion bombardment. The process usually follows the photolithography step in the fabrication of the circuits on the wafer.
The ion implantation process is carried out in an ion implanter, which generates positively-charged dopant ions in a source material. A mass analyzer separates the ions from the source material to form a beam of the dopant ions, which is accelerated to a high velocity by a voltage field. The kinetic energy attained by the accelerated ions enables the ions to collide with and become embedded in the silicon crystal structure of the substrate. Finally, the doped silicon substrate is subjected to a thermal anneal step to activate the dopant ions.
A phenomenon which commonly results from the ion implantation process is wafer charging, in which positive ions from the ion beam strike the wafer and accumulate in the masking layer. This can cause an excessive charge buildup on the wafer, leading to charge imbalances in the ion beam and beam blow-up, which results in substantial variations in ion distribution across the wafer. The excessive charge buildup can also damage surface oxides, including gate oxides, leading to device reliability problems, as well as cause electrical breakdown of insulating layers within the wafer and poor device yield.
Wafer charging is controlled using a plasma flood system (PFS), in which the wafer is subjected to a stable, high-density plasma environment. Low-energy electrons are extracted from an argon or xenon plasma in an arc chamber and introduced into the ion beam, which carries the electrons to the wafer so that positive surface charges on the wafer are neutralized. The energy of the electrons is sufficiently low to prevent negative charging of the wafer.
A typical conventional PFS (plasma flood system) for neutralizing positive charges on ion-implanted wafers is generally indicated by reference numeral 10 in FIG. 1 and includes an arc chamber 12 having a cylindrical chamber wall 14. A single gas inlet opening 18 is provided in the chamber wall 14. A low voltage source 20 generates a typically 3-volt, 200-amp current through a tungsten filament 22 positioned in the chamber interior 13. Pressure inside the chamber interior 13 is maintained at about 5 Torr. Simultaneously, by operation of vacuum pressure induced in the arc chamber 12, a plasma-forming gas such as argon or xenon is introduced into the chamber interior 13 through the gas opening 18, at a flow rate of typically about 1.2 sccm. The filament 22, heated by the low-voltage current from the current source 20, causes thermionic emission of electrons from the flowing gas as the gas contacts the filament 22. The electrons from the gas are electrically attracted to the positively-charged chamber walls 14, which function as an anode. A torroidal magnet 16 generates a magnetic field which causes the electrons to travel in a spiral flight path in the chamber interior 13, and this increases the frequency of collisions between the electrons and the gas atoms, resulting in the creation of additional free electrons. The electrons and positive ions are drawn from the chamber interior 13 through a discharge opening 24, where the electrons, cations and a surrounding plasma 28 enter an ion beam 26 in a beam guide tube 11. The ion beam 26 carries the electrons into contact with a semiconductor wafer (not shown) which was previously subjected to an ion implantation process. Accordingly, the electrons contact the wafer and neutralize positive ions remaining on the surface of the wafer after the ion implantation process.
The system parameters of the PFS 10 are set by recipe and are not variable, so the surface charge condition of the wafer cannot be changed. Consequently, an excessive number of electrons may bombard the surface of the wafer such that the electrons not only neutralize the positive charges on the wafer surface but impart a net negative charge to the wafer. Accordingly, a device is needed for varying the size of the discharge opening 24 in order to achieve a slight positive or neutral charge to the wafer after the ion implantation process.
An object of the present invention is to provide a device for varying the quantity of electrons entering an ion beam in an ion implanter.
Another object of the present invention is to provide a device for preventing excessive negative charging of a substrate during an ion implantation process.
Still another object of the present invention is to provide a device for controlling emission of electrons in a plasma flood system (PFS) for an ion implanter.
Another object of the present invention is to provide a device for mechanically controlling emission of electrons in a PFS for an ion implanter.
A still further object of the present invention is to provide a device for electrically controlling emission of electrons in a PFS for an ion implanter.
Yet another object of the present invention is to provide a mechanical shutter for controlling emission of electrons in a plasma flood system.
A still further object of the present invention is to provide an electron-attracting probe for varying emission of electrons into an ion beam from an arc chamber of a plasma flood system.
In accordance with these and other objects and advantages, the present invention comprises a device for controlling emission of electrons from an arc chamber of a plasma flood system into an ion beam in an ion implanter for implanting ions into a substrate. In one embodiment, the invention comprises a mechanical shutter disposed in a discharge opening between the arc chamber and the plasma guide tube of the implanter. The bore size of the shutter can be selectively varied in order to control the emission of electrons from the arc chamber into the ion beam in the plasma guide tube. In another embodiment, the invention comprises an electron-attracting probe which is disposed in the discharge opening. By varying the strength of the positive charge applied to the probe, the number of emitted electrons which bind to the probe can be varied. This, in turn, controls the number of electrons which enter the ion beam and flood the surface of the substrate.